This invention relates to a method in which aluminum and aluminum alloys are subjected to anodic oxidizing treatment by use of a novel electrolyte to form a good colored oxide film on the surface thereof.
What is called a natural coloring method, in which aluminum is used as an anode for anodic oxidizing treatment to allow the aluminum to be colored, has generally been known in the prior art. Such known methods include Kalcolor of KAISER, U.S.A., in which the electrolyte's main liquid consists of sulfosalicylic acid and sulfuric acid; Duranodic 300 of ALCORE, U.S.A., in which the electrolyte's main liquid consists of sulfophthalic acid and sulfuric acid; Pechiney's method, Switzerland, in which the electrolyte's main liquid consists of sulfomaleic acid and sulfuric acid. The method in which the electrolyte's main liquid consists of an organic acid, results from the substitution of hydrogen which is not a portion of the acid in succinic acid by a sulfonic radical, and sulfuric acid. In the above-mentioned Kalcolor and Duranodic 300 methods, however, the electrolyte is colored due to electrolysis, oxidization or the like, and B.O.D. (Biochemical Oxygen Demand) and C.O.D. (Chemical Oxygen Demand), which are public hazards of the electrolyte, are extremely high in value, (B.O.D.: 45,000 to 50,000 p.p.m.). As a result, to remove such hazards a large expense is required for waste water equipment, which also results in a higher cost for processing.
Further, in the Kalcolor and Duranodic 300 methods and the method using sulfomaleic acid, a longer electrolytic time for coloring is required, the energized quantity is more than 1.5 A/dm.sup.2, and increased electric power is consumed. Moreover, these methods are poor in "color taking", that is, the function of applying uniform color to the entire surface of the treatment members. For example, it is impossible to treat two rows extending between the cathodes. As a result of various experiments, it was found that tasks noted above with respect to the prior art may be solved by using an electrolyte whose main liquid is sulfofumaric acid resulting from the sulfonation of fumaric acid, to which a small amount of inorganic acid or organic acid is added.